Chemical Equilibrium: Le Chatelier՚s Principle and Change in Concentration (For CBSE, ICSE, IAS, NET, NRA 2022)

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Le Chatelier՚s Principle

It states that when a system in equilibrium is disturbed by a change in concentration, pressure or temperature, a ‘net’ change occurs in it in a direction that tends to decrease the disturbing factor.

Change in Concentration

  • Increase concentration of reactant: When the concentration of either nitrogen or hydrogen is increased; a net forward reaction will take place which consumes the added reactant.
  • Increase in the concentration of any product: If the concentration of product ammonia is increased, a net backward reaction would take place to utilize the added ammonia.

Change in Pressure

When the number of moles of products is more than the total number of moles of reactants as in the following system.

  • Increase in total pressure keeping the temperature constant, will cause a decrease in volume. This means that the number of moles per unit volume will increase.
  • A net change will take place in the equilibrium in the direction where the number of moles decrease i.e.. backward direction.
  • When the number of moles of products is less than reactants. As in the following case

Total pressure will bring a net change to the equilibrium in the direction where the total number of moles is decreasing i.e.. to the product side.

Change in Temperature

  • When the temperature is changed, the equilibrium system reacts to nullify the change In heat content.
  • The net change in equilibrium is directed by the exothermic or endothermic nature of reaction.

Exothermic Equilibrium

  • The temperature bring the net change in the equilibrium state in that direction where this extra heat is consumed.
  • Thus, Net change is in the backward direction and some ammonia will decompose producing nitrogen and hydrogen.

Endothermic Equilibrium

  • If the temperature is increased the added heat will be absorbed by the reactant and the net change takes place to the equilibrium in the forward direction.
  • If the temperature in decreased, it will bring a ‘net’ change to equilibrium in the backward direction.

Addition of a Catalyst

  • It does not affect the equilibrium.
  • It helps in achieving equilibrium faster.

Application of Le Chatelier՚s Principle

It can be applied to physical as well as chemical equilibria.

Physical Equilibria

Melting of Ice

The change of ice to water is endothermic process . If the temperature is increased the net change will take place in the forward direction some ice will melt into water.

Vaporization of Water

Increasing the temperature results in more vapour formation (endothermic process) .

Solubility Equilibrium

The process of dissolution can be endothermic or exothermic and depending on which equilibrium is reached.

Chemical Equilibria

Favorable Conditions for Synthesis of Ammonia

During the synthesis of ammonia such conditions are maintained which favour the ‘net’ forward reaction namely low temperature and high pressure.

Formation of

It is exothermic process and Formation of will be favored by high pressure and low temperature in the presence of a catalyst.

Formation of

It is endothermic and the reaction is not affected by pressure changes and is favored at high temperature.

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